Currently, most high voltage switching circuits are designed using silicon-based transistors, such as Si MOSFETs or IGBTs. A schematic diagram of a Si power MOSFET is shown in FIG. 1. As indicated, the source and gate electrodes 10 and 11, respectively, are on one side of the semiconductor body 13, and the drain electrode 12 is on the opposite side.
Prior to inserting the transistor of FIG. 1 into a discreet circuit, the transistor is encased in a package. Schematic examples of conventional transistor packages are shown in FIGS. 2 and 3. Referring to FIG. 2, the package includes structural portions, such as the case 24 and package base 23, as well as non-structural portions, such as leads 20-22. The case 24 is formed of an insulating material, the package base 23 is formed of a conducting material, the gate lead 21 is formed of a conducting material and is electrically connected to the gate electrode 11 of the transistor, the drain lead 22 is formed of a conducting material and is electrically connected to the package base 23, and the source lead 20 is formed of a conducting material and is electrically connected to the source electrode 10 of the transistor. As shown, the transistor is mounted directly to the package base 23 with drain electrode 12 in electrical and thermal contact to the package base 23. The drain electrode 12 and package base 23 are connected such that their electric potentials are about the same under all bias conditions and the heat generated during operation can easily dissipate to the package base. Drain lead 22 and drain electrode 12 are thereby electrically connected, since both are electrically connected to the package base 23. A metal bond wire 31 can form an electrical connection between the gate electrode 11 and the gate lead 21. Similarly, source lead 20 can be electrically connected to source electrode 10 via bond wire 30.
The package in FIG. 3 is similar to that of FIG. 2, except that the package case 26 is formed of a conducting material, so the package base 23 and case 26 are at the same electrical potential (i.e., they are electrically connected). For this package, the source and gate leads 20 and 21, respectively, are electrically isolated from the package case 26, while the drain lead 22 is electrically connected to the case. Drain electrode 12 is electrically connected to the package base 23, gate lead 21 is electrically connected to the gate electrode 11 of the transistor, and source lead 20 is electrically connected to the source electrode 10 of the transistor.
As shown in FIG. 4, when the packaged transistor of FIG. 2 is used in a circuit assembly or on a circuit board, it is typically mounted on a heat sink 27 with an insulating spacer 28 between the package base 23 and the heat sink 27 to form transistor assembly 25. The insulating spacer 28 is made thin to allow heat generated by the transistor to transfer to the heat sink through the insulating spacer 28. However, the insulating spacer 28 has at least a minimum thickness, because decreasing the thickness of the insulating spacer 28 increases the capacitance between the package base 23 and the heat sink 27. In many cases, the heat sink 27 is connected to a circuit ground, hence the capacitance between the drain and the heat sink translates to a capacitance between the drain and ground. When the heat sink is not connected to the circuit ground, there is typically a large capacitance between the heat sink and the circuit ground, since the surface area of the heat sink is typically much larger than that of the transistor. This again results in a large total capacitance between the drain and the circuit ground.
FIG. 5 shows a circuit schematic of the transistor assembly 25 of FIG. 4 after it is mounted on a circuit assembly or circuit board, and its source is connected to ground 33. Capacitor 32 represents the capacitance between the package base 23 and the circuit ground, i.e., the capacitance between drain electrode 12 and the circuit ground. During operation of the transistor assembly 25, the charging and discharging of capacitor 32 not only causes severe switching losses but also results in the emission of electromagnetic radiation, also known as electromagnetic interference (EMI), thereby degrading the performance of the circuit. Capacitor 32 can cause common-mode AC currents to flow to ground through a path outside the desired signal path. The larger the capacitance of capacitor 32, the higher the switching loss and the intensity of common-mode EMI emission, which results in degradation of electrical performance. Hence, there is a trade-off between improved electrical performance, which may require a thick insulating spacer 28, and dissipation of heat produced by the transistor during operation, which may require a thin insulating spacer. Device and package configurations are desirable for which both switching losses and EMI can be adequately mitigated and simultaneously heat can be adequately dissipated when the device is used in a circuit such as a high voltage, high power switching circuit.